Automotive accidents are a major cause of loss of life and dissipation of resources in substantially all societies in which automotive transportation is common. It is estimated that over 10,000,000 people are injured in traffic accidents annually worldwide and that of this number, about 3,000,000 people are severely injured and about 400,000 are killed. A report “The Economic Cost of Motor Vehicle Crashes 1994” by Lawrence J. Blincoe published by the United States National Highway Traffic Safety Administration estimates that motor vehicle crashes in the U.S. in 1994 caused about 5.2 million nonfatal injuries, 40,000 fatal injuries and generated a total economic cost of about $150 billion.
Lack of driver attention and tailgating is estimated to be a cause of about 90% of driver related accidents. Methods and apparatus that would alert a driver to a potential crash and provide him or her with sufficient time to undertake accident avoidance action would substantially moderate automotive accident rates. For example a 1992 study by Daimler-Benz indicates that if passenger car drivers have a 0.5 second additional warning time of an impending rear end collision about 60 percent of such collisions can be prevented. An extra second of warning time would lead to a reduction of about 90 percent of rear-end collisions.
Various systems collision waning/avoidance systems (CWAS) exist for recognizing an impending collision and warning a driver of the danger. U.S. Pat. No. 5,529,138, describes a CWAS that uses a laser radar to determine distance and relative velocity to determine a time to collision of a vehicle with an object. U.S. Pat. No. 5,646,612 describes a CWAS system comprising a laser radar and an infrared (IR) camera. A processor determines a time to collision (TTC) of a vehicle with an object responsive to signals provided by the laser radar and whether the object is a human, an animal or an inanimate object responsive to image data provided by the IR camera. The system operates to warn a driver of art impending collision with art object based on the TTC and kind of object “and properly performs deceleration and braking operations based on a position of the object and a speed of the vehicle is disclosed”. The disclosures of the above noted U.S. patents are incorporated herein by reference.
Laser radar systems are relatively complicated systems that are generally expensive and tend to suffer from narrow field of view and relatively poor lateral resolution. PCT Publication WO 01/39018, the disclosure of which is incorporated herein by reference, describes a CWAS that comprises a camera and a processor for processing image data provided by the camera. The camera provides images of an environment in which a vehicle is located and the processor determines a TTC of the vehicle with an obstacle by processing, optionally only, image data provided by the camera. The processor determines the TTC responsive to scale changes in the size of the obstacle as imaged in the images under the assumption that the relative velocity between the vehicle and the object is constant.